基于超声相控阵的耐候钢接头疲劳裂纹动态监测

借助超声相控阵技术对耐候钢对接接头开展疲劳失效过程动态监测. 基于超声波探头的信号特征,研究其扇形扫描反射过程,建立实时扫查方案, 并对10 mm厚的耐候钢对接接头实施实时监测.结果表明,当疲劳寿命为5 × 104次时,相控阵检测到多个裂纹从对接接头焊趾部位萌生,并沿着板厚扩展,当疲劳寿命超过3.5 × 105次时,裂纹开始快速扩展. 与疲劳试验断口对比发现,基于相控阵检测得到的裂纹尺寸与试验结果基本一致,验证了相控阵裂纹动态检测的准确性. 根据裂纹深度a、裂纹长度2c与循环次数N关系,明确了裂纹动态演化行为,并获得中厚板耐候钢对接接头表面裂纹的扩展演化规律.     

ADB610钢焊接接头概率疲劳裂纹扩展分析

采用焊条电弧焊方法对ADB610钢进行焊接,对焊接接头进行疲劳裂纹扩展试验,研究其概率疲劳裂纹扩展规律.采用两步七点递增多项式拟合方法,计算了一系列相同裂纹长度下母材区、热影响区和焊缝区的对数裂纹扩展速率方差和存活率分别为50%,90%,95%,99%,99.9%的裂纹扩展速率.结果表明,总体上母材区的裂纹扩展分散性最小,热影响区其次,焊缝区分散性最大;而且总体上存活率相同时,在裂纹扩展的初期,母材区的裂纹扩展速率最快,焊缝区的裂纹扩展速率最慢;在裂纹扩展末期,三区域的裂纹扩展快慢相当;随着存活率增大,三区域裂纹扩展快慢差别不大时所对应的裂纹长度减小.     

高强度桥梁钢焊接接头疲劳裂纹扩展速率的研究

利用高频疲劳试验机对高强度桥梁钢的母材、热影响区和焊缝的疲劳裂纹扩展速率进行了测定,并结合疲劳扩展断口分析各部位疲劳裂纹扩展快慢的原因。结果表明,高强度桥梁钢焊接接头中焊缝裂纹扩展速率最低,抗疲劳裂纹扩展能力最高;母材疲劳裂纹扩展速率最高,抗疲劳裂纹扩展能力最差;热影响区介于二者之间。焊缝的疲劳断裂模式是以穿晶断裂为主,母材是以沿Z字形路径扩展的沿晶断裂为主,热影响区则介于两者之间。     

GH536高温合金焊接接头疲劳裂纹萌生与扩展原位试验研究

采用原位疲劳试验方法,实时观察了GH536焊接接头疲劳裂纹的萌生和扩展行为,从而揭示了GH536焊接接头疲劳裂纹的萌生和扩展机制：疲劳加载过程中,位错沿滑移带在晶界前沿塞积,晶界阻碍位错运动,裂纹沿滑移带开裂,萌生疲劳裂纹;疲劳裂纹扩展初期,受单滑移的交替作用,裂纹呈“Z”字型向前扩展,随后裂纹的扩展逐渐以主应力控制为主,垂直于加载方向、平直向前扩展;GH536合金焊接接头组织中的晶界和碳化物会阻碍疲劳裂纹的扩展。     

TC17钛合金电子束焊接接头的疲劳裂纹扩展规律及疲劳剩余寿命

首先通过试验取得母材及焊接接头的疲劳裂纹扩展速率,然后结合TC17钛合金电子束焊接接头CTOD试验结果及裂纹容限计算值,以估算其疲劳剩余寿命.结果表明:在低应力水平或低△K下,TC17电子束焊缝的da/dN数据与母材的基本相当;然而随着应力水平的增加,焊缝的da/dN值越来越大.在初始裂纹尺寸相同的情况下,TC17合金电子束焊缝与母材疲劳裂纹扩展寿命曲线存在交叉点.当应力幅大于交叉点应力幅时,TC17母材疲劳裂纹扩展到临界裂纹尺寸的剩余寿命要高于相应焊缝的剩余寿命;当应力幅小于交叉点应力幅时,TC17母材扩展到临界裂纹尺寸的剩余寿命要低于相应焊缝的剩余寿命.     

管线钢疲劳裂纹扩展速率与疲劳寿命关系的研究

通过对管线钢表面缺陷的断裂力学分析,建立了用管线钢的疲劳裂纹扩展数据估算含初始缺陷的管道在运行条件下的疲劳寿命的数学计算方法．用该方法计算了6种管线钢在给定运行条件下的疲劳寿命,并进行了模拟实际条件的疲劳实验,表明管线钢疲劳寿命的计算值与实验值之间存在良好的线形关系,即在疲劳裂纹扩展实验结果的基础上,计算管线钢在实际运行条件下的疲劳寿命是可行的．同时讨论了管线钢的组织对其疲劳性能的影响．     

高速列车用高强铝合金焊接接头疲劳裂纹的扩展特性

使用疲劳试验机测试A7N01铝合金焊接接头的疲劳裂纹扩展速率,采用光学显微镜对焊接接头的显微组织进行分析,使用扫描电镜对疲劳断口形貌进行研究。结果表明:母材的显微组织为时效状态的α(Al)基体与粗大一次相、弥散二次强化相以及少量的夹杂物构成的轧制状态组织;热影响区的显微组织为α(Al)基体以及少量未固溶的一次相组织,焊缝的显微组织为α(Al)基体与离异共晶组织。焊接接头的显微组织以及第二相粒子尺度不同导致了疲劳裂纹扩展特性不同。随着应力比的增加,疲劳裂纹扩展速率均呈增大趋势。裂纹尖端塑性区内的第二相粒子与基体的不协调塑性变形可促进疲劳裂纹的扩展,影响疲劳裂纹的扩展速率。     

2Cr13钢的疲劳裂纹萌生与扩展行为

通过对2Cr13钢进行疲劳裂纹萌生与扩展速率试验,获得该钢在两种不同调质处理下的疲劳裂纹萌生寿命Ni的回归方程、疲劳裂纹萌生门槛值以及裂纹扩展速率da/dN表达式。结果表明,与970℃淬火+710℃回火×6h工艺相比,2Cr13钢在970℃淬火+640℃回火×5h处理后的缺口疲劳裂纹萌生抗力有所改善;当应力强度因子较低时,在640℃×5h回火时的2Cr13钢的疲劳裂纹扩展抗力稍好一些;当应力强度因子较高时则反之。     

疲劳短裂纹扩展中的混沌现象

在低周疲劳的短裂纹扩展中 ,利用非线性动力学理论 ,分析 Hobson的短裂纹扩展公式 [1 ] ,发现在一定条件下会出现分岔、混沌现象。计算机模拟试验显示 ,在混沌区域内 ,即使初始裂纹尺寸相差很小 ,在经过若干次的应力循环后 ,两者之间会相差很大。如果假设裂纹是以单裂纹穿晶形式扩展 ,发现参数在许多取值区间会发生分岔、混沌现象。由此可知 ,短裂纹扩展的分岔和混沌 ,是导致疲劳试验结果分散性大的原因之一。     

Fatigue properties of welded joints using steel with high resistance to fatigue crack growth

It has been generally recognized that the fatigue life of welded joints is little influenced by the strength of steels owing to the high-stress concentration and the tensile residual stress near the weld toe. In this paper, improvement of the fatigue life of welded joints using steel with high resistance to fatigue crack growth (ferrite/martensite (F/M) steel) is investigated. F/M steel has a microstructure with an elongated and banded martensite phase distributed in a ferrite matrix and a fatigue crack growth rate of about one-half to one-tenth in the thickness direction, compared with conventional steel. As a result, the fatigue life of an out-of-surface gusset-welded joint increases with the decrease of the fatigue crack growth rate. The fatigue life of welded joints using F/M steel with the highest resistance to fatigue crack growth increases to about twice that of joints using conventional steel. Whereas the fatigue crack growth rate decreases significantly, the fatigue life of welded joints increases only slightly. This can be attributed to the stress ratio independent of the fatigue crack growth rate. In other words, the fatigue crack growth rate of F/M steel increases with the increase of the stress ratio, approaching that of conventional steel. In the case of welded joints, even if a fatigue test is carried out at a low-stress ratio, the region near the weld toe is under a high-stress ratio due to tensile residual stress. Therefore, improvement of the fatigue life of welded joints becomes comparatively small so that the effect of fatigue crack retardation of F/M steel decreases.     

Fatigue crack growth of bainitic rail steel welds

Abstract

The objective of the present study is to develop and evaluate novel weld repairs of bainitic rail steel defects, such as detail fracture, induced by in service loading. Slots were machined in the bainitic rail steel to simulate the removal of service defects. Multipass gas metal arc welding was used to fill and repair the slots. Three-point bend tests of the parent and welded steels revealed that the flexural weld efficiency was 75%. Fatigue crack propagation (FCP) tests were performed on specimens from the welded joints and compared with the parent materials to determine the mechanical integrity of the slot repairs. The fatigue lifetime and FCP kinetics were similar for the parent and slot welded bainitic steels, indicating similar resistance to FCP. The fatigue fracture surface morphology of the parent and welded bainitic rail steels both revealed mostly a ductile fracture mechanism in all the three FCP stages.     

Fatigue crack growth of titanium alloy joints by electron beam welding

Electron beam welding（EBW） has been widely used in the manufacture of titanium alloy welded blisk for aircraft engines. Based on fatigue crack growth tests on titanium alloy electron beam welding（EBW） joints, mechanism of fracture was investigated under scanning electron microscope（SEM）. The results show that fatigue crack growth rate increases as the experimental load increases under the same stress ratio and stress intensity factor range. At the beginning of crack growth, the extension mechanism of fatigue crack is the typical mechanism of cleavage fracture. In the steady extention stage, crack extends along the weld seam firstly.Then, crack growth direction changes to extend along the base metal. The extension mechanism of fatigue crack in the weld seam is the main mechanism of cleavage fracture and the extension mechanism of fatigue crack in the base metal is the main extension mechanism of fatigue band. In the instantaneous fracture stage, the extension mechanism of fatigue crack is the typical dimple-type static fracture mechanism.Crack growth was simulated by conventional finite element method and extended finite element method.     

基于信息熵的铝合金焊接接头疲劳寿命分析方法

建立了基于信息熵的铝合金焊接接头疲劳数据分析模型,通过对比分析各决策属性的信息分熵,研究了应力集中、板厚与载荷类型等因素对焊接结构疲劳破坏的定量贡献. 结果表明,三种应力类型疲劳数据分布的信息总熵分别为0.997 7,0.910 0和0.817 9,基于权重信息熵的降低的总趋势与S-N曲线标准偏差越来越小的情况存在一致性. 对于基于结构应力的疲劳数据分布,膜应力集中系数(SCFm)基于权重的信息分熵为0.675 4,对于结构应力疲劳数据分布起主要作用;对于等效结构应力,应力比的基于权重的信息分熵为0.622 3,说明应力比对于等效结构应力疲劳数据分布起重要作用.     

P92钢焊接接头蠕变损伤与裂纹扩展数值模拟

高温焊接接头由于蠕变损伤而提前失效的案例频频发生,准确预测焊接接头的蠕变损伤和裂纹扩展行为对于保证高温装备的结构完整性具有重要意义.文中基于延性耗竭模型并结合有限元方法,考察了结构因素对厚壁圆管焊接接头蠕变失效行为的影响.结果表明,蠕变裂纹萌生/扩展行为受热影响区宽度影响,细晶热影响区宽度对蠕变裂纹的萌生时间影响不大,但会改变裂纹萌生位置;相比之下,粗晶热影响区的宽度变化对裂纹萌生时间影响略大.不同坡口形式展现出不同的裂纹萌生/扩展行为,而X形坡口是四种坡口形式中的较优选择.     

基于结构应力的焊接接头疲劳分析

基于结构应力法对来自相关文献中的16Mn钢十字非承载焊接接头、十字承载接头和纵向立板角焊缝接头的疲劳强度的试验数据进行了分析.推导了适合有限元计算的离散结构应力计算公式,得到焊接接头的结构应力集中系数.证实了该方法具有网格划分不敏感性,给出了应用结构应力法进行疲劳评定时适合上述各类接头的结构应力S-N曲线.结果表明,同名义应力法相比较,采用结构应力表示的焊接接头疲劳试验数据其分散性较小.     

Fatigue crack growth in the welding nugget of FSW joints of a 6060 aluminum alloy

Friction stir welded butt joints were performed on 8 mm thick sheets made of AA6060 T6 aluminum alloy by means of a CNC machine tool, at feed rates between 117 and 683 mm/min and tool rotational speed between 838 and 1262 rpm. Tensile tests, metallographic analyses and micro-Vickers tests were carried out to evaluate the mechanical properties of the joints as a function of the process parameters. The fatigue behavior was studied by means of crack growth tests performed according to ASTM E647 standard on CT specimens, with propagation in the middle of joint along the weld nugget. The results show the influence of welding process parameters on mechanical properties and fatigue behavior. Reduction of UTS of about 20–30% with respect to base material occurred with rupture in the softened zone of welding, usually HAZ. In this range, slight variations of joint efficiency were observed with f/S ratio, while the width of the softening area increases for decreasing values of this parameter. Fatigue crack growth was always slower than that in the base material at low ΔK below 12 MPa m^1/2. The effect of non-optimal welding parameters was evident at intermediate and high ΔK, due to defects, such as tunnels, that cause dramatic increase of propagation rate up to five times higher than the base material.     

Fatigue crack growth behavior of laser-processed 304 stainless steel in air and gaseous hydrogen

Fatigue crack growth test was performed to evaluate fatigue behavior of 304 stainless steel specimens with or without laser processing (welding and surface treatment) in air and gaseous hydrogen. As the crack propagation normal to the laser welding or scan direction, the laser-processed specimens exhibited a higher resistance to crack growth in the low stress intensity factor range (ΔK) than the as-received steel plates regardless of testing environments. However, the marked retardation of crack growth behavior vanished for welded specimens subjected to a 850 °C/h stress relief treatment or with a shorter distance from notch tip to the weld centerline in the test.Fatigue-fractured appearance of the steel plate tested in air was composed of mainly transgranular fatigue fracture and some flat facets, along with a small amount of intergranular fracture. While quasi-cleavage fracture and few twin boundary separations were observed for the same specimen in hydrogen. On the other hand, the lower crack growth rate of laser-processed specimens in both air and hydrogen was accompanied with rubbed areas on the fracture surfaces. It was found that the extent of quasi-cleavage fracture was related to the formation of strain-induced martensite, which would contribute to an increased fatigue crack growth rate of all specimens in gaseous hydrogen.